The present invention relates to the field of inkjet printing. More particularly, the present invention provides a radiation curing system and method for use with inkjet printers using radiation-curable inks.
Inkjet technology and processes for printing radiation-curable inks are known, with the most-widely investigated inks being those that are cured upon exposure to ultraviolet (UV) radiation. Radiation-curable inks offer advantages such as no volatile organic compound emissions and increased durability as compared to solvent/aqueous inks.
In spite of these advantages, radiation curable inks still suffer from the problems associated with the low viscosity state in which they are applied to a substrate. That low viscosity can be the source of a number of problems including control over dot gain (size, shape, etc.), color mixing and pooling caused by applying two inks of different colors at the same location, and coalescence. Some of these disadvantages may be more problematic when the inks are applied to impermeable substrates as opposed to porous substrates.
Various attempts to address the problems with radiation-curable inks face a number of additional difficulties. Among the difficulties are the need to move the inkjet printhead or heads across the substrate because of the excessive cost associated with providing a row of printheads sufficiently large to print on a substrate in a single pass. Printhead carriages and the mechanisms used to move them across a substrate are carefully designed to provide acceptable printing speed and accuracy. Mounting radiation sources directly on the printheads or their carriage assemblies typically results in decreased image quality and/or printing speed due to the additional mass moved during printing. It may also result in very limited or no flexibility in selecting a delay interval between printing the ink and curing it.
Attempts to address the issues of added mass on inkjet printhead carriages are discussed in International Publication No. WO 97/04964 (Caiger et al.) in which the radiation from a stationary source is delivered to the printhead carriage using an optical fiber or by the use of mirrors. In all instances, however, the radiation is delivered to the printed substrate downstream from the printing location, where downstream is in reference to the movement of the substrate relative to the printhead (which traverses the width of the substrate during printing). Depending on the speed of the substrate, the delay between printing and curing may be excessive, causing problems in dot gain control, color mixing, and coalescence.
The present invention provides methods and systems for curing radiation-curable inks printed on substrates using inkjet printheads. The methods and systems include radiation sources that are either integral with a printer or that can be added to an existing printer. In either case, the radiation source preferably moves independently of the printhead to provide the desired electromagnetic curing energy to the printed ink.
As used in connection with the present invention, xe2x80x9ccuringxe2x80x9d may include partially or completely curing the radiation-curable ink. In some instances, the initial dose of radiation may only partially cure the ink with a later dosage provided to complete the curing process.
An advantage of the invention is that the radiation source and the printhead are mounted separately and move independently such that the mass of the printhead is not significantly affected. As discussed above, adding mass to a printhead can result in decreased image quality and/or print speed.
One advantage of the invention of at least some embodiments of the invention is the ability to control or select a curing delay, i.e., the time interval between printing of the radiation-curable ink on the substrate and curing the ink.
Another advantage of some embodiments of the invention is the ability to retrofit existing printers with a radiation source for use with radiation-curable inks.
In one aspect, the present invention provides a method for printing and curing a radiation-curable ink by providing a printhead movable in first and second directions along a printing axis and providing a radiation source movable in first and second directions along a curing axis, wherein the curing axis is parallel to and offset from the printing axis. A substrate (having first and second sides defining a substrate width therebetween) is moved in a travel direction relative to the printhead, wherein the travel direction is transverse to the printing axis. The method also includes moving the printhead between the first and second sides of the substrate along the printing axis while printing a radiation-curable ink on the substrate with the printhead and curing the radiation-curable ink by following the movement of the printhead with the radiation source while activating the radiation source. The radiation source follows the printhead during movement of the printhead in the first and second directions along the printing axis. In addition, the printhead passes between the radiation source and the substrate proximate the first and second sides of the substrate.
In another aspect, the present invention provides a method for printing and curing a radiation-curable ink by providing a printer including a printhead movable in first and second directions along a printing axis, and attaching a radiation source to the printer, the radiation source movable in the first and second directions along a curing axis, wherein the curing axis is parallel to and offset from the printing axis. A substrate (having first and second sides defining a substrate width therebetween) is moved in a travel direction relative to the printhead, wherein the travel direction is transverse to the printing axis. The method also includes moving the printhead between the first and second sides of the substrate along the printing axis while printing a radiation-curable ink on the substrate with the printhead and detecting movement of the printhead. The radiation-curable ink is cured by following the movement of the printhead with the radiation source while activating the radiation source. The radiation source follows the printhead during movement of the printhead in the first and second directions along the printing axis. In addition, the printhead passes between the radiation source and the substrate proximate the first and second sides of the substrate.
In another aspect, the present invention provides a radiation-curable ink printing system including a substrate transport apparatus defining a substrate travel direction, wherein the substrate transport apparatus includes first and second sides defining a width therebetween. The system also includes a printhead movable in first and second directions along a printing axis that is transverse to the substrate travel direction and a radiation source independent from the printhead, the radiation source being movable in the first and second directions along a curing axis that is parallel to and offset from the printing axis. The printing axis is located between the substrate transport apparatus and the curing axis such that the printhead passes between the radiation source and the substrate transport apparatus during printing.
In another aspect, the present invention provides a radiation-curable ink curing system adapted for attachment to a radiation-curable ink printer including a substrate transport apparatus defining a substrate travel direction, wherein the substrate transport apparatus has first and second sides defining a width therebetween. The printer also includes a printhead movable along a printing axis that is transverse to the substrate travel direction. The system includes a radiation source movable in first and second directions along a curing axis that is parallel to and offset from the printing axis. The radiation source is attached to the printer such that the printing axis is located between the substrate transport apparatus and the curing axis, wherein the printhead passes between the radiation source and the substrate transport apparatus during printing.
These and other various features and advantages of the invention are described below with reference to various illustrative embodiments of the invention and examples of the invention.